Sintered rare-earth cobalt magnets comprising mischmetal plus cerium-free mischmetal

ABSTRACT

A sintered permanent magnet of compacted and sintered particles of a cobalt-rare-earth compound with the total rare-earth element content being within the range of 32 to 42% by weight, which total rare-earth content consists essentially of, in weight percent, 25 to 75% mischmetal and 25 to 75% cerium-free mischmetal; alternately, up to 30% samarium may be substituted for a like percent of the mischmetal, cerium-free mischmetal or a portion of each with respect to the total rare-earth element content. Preferably, the mischmetal and cerium-free mischmetal are present in approximately equal amounts either alone or in combination with samarium up to 30% by weight.

United States Patent 1 1 Ratnam 1 1 SINTERED RARE-EARTH COBALT MAGNETS COMPRISING MISCHMETAL PLUS CERIUM-FREE MISCHMETAL [75] Inventor: Devineni V. Ratnam, Scott Township, Allegheny County. Pa.

[73] Assignee: Crucible Inc., Pittsburgh, Pa.

1 22] Filed: Mar. 4, 1974 [211 App]. No.: 448,141

[52] US. Cl. 148/3157; 75/152: 75/200; 148/101; 148/105 [51] Int. C1. ..H01F1/04 {58] Field of Search 148/3157. 101. 103. 105; 75/84. 200. 152

[56] References Cited UNITED STATES PATENTS 3.424.578 1/1969 Strnat et a1. 75/200 3.655.463 4/1972 Benz .7 148/101 3.682.716 8/1972 Martin 148/3157 3.821.035 6/1974 Martin 148/3157 OTHER PUBLICATIONS Strnat. K; Cobalt-Rare Earth Alloys as Promising Per 1 Nov. 11, 1975 manent Magnet Materials. in Cobalt. 16, Sept. 1967. pp. 133-143.

Ronson Metals Corp. Rare Earth Metals. No. 9. (4-67). 2 p.

Buschow. K. et a1; Perm. Mag. Materials of Cobalt Rare Earth Compounds. in Zeit. Fur, Ang. Phys. 26 1969. pp. 157-160.

Prirmrry E.\' mziucrWalter R. Satterfield [57] ABSTRACT A sinterecl permanent magnet of compacted and sinterecl particles of a cobalt-rare-earth compound with the total rare-earth element content being within the range of 32 to 43% by weight. which total rare-earth content consists essentially of. in weight percent. 25 to 75% mischmetal and 25 to 75% cerium-free mischmetal; alternately. up to 30% Samarium may be substituted for a like percent of the mischmetal. cerium-free mischmetul or a portion of each with respect to the total rare earth element content. Preferably. the mischmetal and cerium-free mischmetal are present in approximately equal amounts either alone or in combination with samarium up to 307: by weight.

3 Claims. N0 Drawings SINTERED RARE-EARTH COBALT MAGNETS COMPRISING MISCHMETAL PLUS CERlUM-FREE MISCHMETAL Permanent magnets, and particularly those of the present invention, are used in applications such as electrical motors and generators. biasing magnets for relays, microphones and telephones. microwavev tubes and loudspeakers.

It is known that by the use of rare-earth cobalt compounds and particularly SmCo,-,, permanent magnets characterized by improved magnetic properties, particularly improved energy product and coercive force may be produced. Typically, magnets of this type are made by melting together the desired amounts of the components, such as Samarium and cobalt, under a protective noble gas atmosphere or vacuum. The melting may be achieved by are melting on a cold copper hearth, induction melting in pure alumina crucibles or levitatiori metling. The resulting rare-earth cobalt alloy is then crushed and ground in a ball mill, vibratory mill or a fluid energy jet mill to form particles, with the particles typically being smaller than 50 microns, preferably 2 to microns. Thereafter the powder is placed in a container and subjected to magnetic alignment. It is then compacted either by pressing, sintering or a,combination thereof to achieve final density.

lt has been found that with rare-earth cobalt magnet materials, and particularly SmCo magnet material. it is difficult to achieve the desired high density, c.g., substantially greater than 907: of theoretical density, with out resorting to high sintering temperature and long times at temperature. Even when increased sintering Samarium is substituted for a portion of the mischmetal I and used in combination therewith in a cobalt rarecarth element compound, although the magnetic properties are increased with increased amountsiof samarium, the material costs of the magnet are substantially increased. Below samarium as a fraction of the total rare-earth content of the magnet, the. properties remain inferior to a degree that is economically unattractive relative to the total raw material cost.

It is accordingly a primary object of the, present invention to provide a thermally and magnetically stable magnet of a cobalt-rare-earth compound wherein improved magnetic properties are achieved principally by the use of relatively inexpensive rare-earth element additions and which does not require the sintering times at temperature to achieve densification at which the magnetic properties, and particularly the coercive force. are diminished.

This and other objects of the invention as well as a more complete understanding thereof maybe obtained from the following description and specific examples.

Broadly in the practice of the invention compacted. sintered permanent magnets are produced from a combination of particles of cobalt-rare-earth (C oRE) compounds. The sum of the rare-earth elements employed in the cobalt compound ranges from 32 to 4271 by weight. The total amount of rare-earth elements constitutes 25 to by weight mischmetal (MM) and 25 to 757: by weight cerium-free mischmetal (FM). Alternately. up to 30% by weight samarium may be substituted for either the mischmetal, the cerium-free mischmetal or both. Preferably. 8 to 30% by weight samarium is substituted. Preferably. the mischmetal and cerium-free mischmetal are present in approximately equal amounts. The CoRE particles are processed in a conventional manner by magnetically aligning. cold pressing to form a compact and sintering in a nonoxidizing atmosphere. Sintering is effected at a temperature of about l850 to 200UF for about It) to 60 minutes. If desired, after sintering. an homogenization treatment at an elevated temperature of for example about l6()() to l900F may be employed.

The CoRE particles may be produced by a single melt practice wherein the composition constituting cobalt in combination with mischmetal and cerium-free mischmetal. and samarium if used, is produced in a single melt. solidified and then comminuted to form the particles for compacting and sintering. Alternately. a dual melt practice may be employed wherein CoMM and (0PM compounds are produced in separate melts. and particles therefrom are mixed to achieve the desired particle mixture for processing. This can also be the Case with magnet material having samarium substituted for a portion of the mischmetal and/or ceriumfree mischmetal contents. in such instance a separate heat of CoSm with Sm constituting St) to thereof may be produced. solidified and comminuted to form particles that may be mixed with CoMM and CoFM particles or CoMMFM particles. In this instance with the CoFM and CoMM compounds or the CoMMFM compound. the total rare-earth content of each compound would be within the range of 30 to 40% by weight with the balance cobalt. It has been found that by substituting CoSm for a portion of the particle mixture improved coercive force is achieved. With respect to the mischmetal and cerium-free mischmetal employed in the practice of the invention typical mischmetal (MM) would have a typical composition. in weight percent. Ce 44 to 50%. La l) to 32%. Nd I} to 20%, Pr 4.4 to 5.471 and other rare earths and residual elements up to about 2%, typical cerium-free mischmetal (FM from which the cerium has been substantially removed. would have a typical composition, in weight percent. Nd 42%, La 40%. Pr 14%, Gd 3% and cerium and other rare earths and residual elements 371. Since both MM and FM contain Sm, it is to be understood that reference hereto to Sm additions does not include Sm which is present in the magnet because of its presence in the MM and FM additions constituting the magnet.

Table l lists the magnet composition as well as the properties for various magnets produced in accordance with the invention, which constitute Examples 1 through 6, and prior art magnets, which constitute Examples 7 through l0.

TABLE I Composition (Wtfii Sta- Intrinsic Cc- Alignbility Energy Free ing Sintering Properties Param- Product Misch- Misch- Simian Co- Field Temp. Time B, H...- H (BH etcr H B Ex. metal metal ium* halt ()ersted Min. Gauss ()crsted Ocrsted ()crstcd MGOe H /B MGOe l-l l4.7 ll 9.6 62.4 20.000 i930 l 8.200 l-l.lll0 7.650 8.400 I00 L01 69.0 Z-l ll) I39 9.7 01.5 20.000 l920 60 7.550 l0.000 7.450 9.000 l4.l L19 68.0 3 l l3. llli 8.4 (14.0 7000 U420 00 7.X00 l L900 7.000 7.000 l-l,li 0.9 54.5 4-D l3 8 llit 8.4 6-3.0 4.000 I915 30 7.370 H.000 6.700 (1.900 I17 0.93 5L0 5-1) lli'l llli 90 63.4 4.000 I920 45 7.040 l [.400 6.80) 9.600 ll: L36 67.5 6-D l-l 7 I47 K6 6: 0 4.000 I910 60 6.700 l4.ll00 0.400 lZ.000 l0) l .79 80.0 7-l :6 l 0 I0 6. .0 60.000 l8}: .0 (.700 H.700 0.200 7.500 I09 1.! 50.0

[0 l00.000 X-l 37 3 0 3.7 0-4.0 00.000 1831 30 7.000 9.600 6.300 0.500 l 1.4 0.93 45.4

to |00 l)00 I: I 0 I17 64.5 60.000 I377 30 7.630 8.500 5.2450 4.600 I} h 0.60 35.3 -)-l [u l00.000 l0 sintered product of No. 9. sintered at 8.440 6.300 5.400 4.000 16.6 0.47 33.8

WIZF for 30 min and heat aged at I65IF for 10 hours.

I isostatic Pressing I) Die Prcwng Samarium content shown is EXAMPLE l A 60-gram quantity of the composition listed in Table l as Example I was melted in a tungsten arc inert gas chamber. The material was crushed and screened to minus mesh particle size which material was ball milled to a particle size range of 2 to 10 microns. Particle mixture included (0 M M ||.4.= Particles 901150 tuting 17.5% MM. l5.87r FM and 6677: Co which was mixed with I67: by weight of CoSm wherein Co was present in an amount of about and Sm in an amount of about 60% to result in a final particle mixture of MM 14.77r. FM l3.37'. Sm 9.6%. Co 62.4%. This powder mixture was oriented in a rubber tube in the presence of a 20,000 0e magnetizing field and compacted isostatically at a pressure of l20.000 psi. The compact was sintered at I930F for ten minutes in helium atmosphere and cooled to l600F in about five minutes and oil quenched to room temperature. It may be noted from Table I that the magnet had good remanence. coercive force and an energy product of 16 MGOe.

EXAMPLE 2 ln Example 2 the composition reported in Table l was produced by providing particles of Co MM., -,FM., from a l000-gm. induction melt made from a l67? FM. 16% MM and Co 68%. which mixture was combined with l37r CoSm with Co constituting 25% and Sm 75% of the compound. The mixture was oriented in a rubber bag at the magnetic field of 20,000 Oe and isostatically compacted at a pressure of l20.000 psi. Sintering was conducted at l920F for 60 minutes in a helium atmosphere. Thereafter the sintered compact was furnace cooled to l500F in ten minutes and rapidly cooled from lS00F using a helium gasjet (helium gas jet cooling would be somewhat less rapid than an oil quench).

EXAMPLE 3 The magnet material of Example 3 was made starting from an I l-pound induction melt of the composition: MM 167:. FM l67r and Co 68%. Powder of 2l0 micron size was obtained by ball milling and to this powder 14% by weight of sinter aid of Sin (60%) Co (40%) was added. The compact was produced by isostatic in addition to the limited amounts ut'Sni contained in Mischmutal and cerium-tree Misch metal pressing at [20.000 psi with an aligning field of 7,000 Oe. The compact was sintered at l920F for one hour and cooled in a jet of helium gas from approximately l450F. The resulting magnet had an energy product of 14.8 MGOe with other magnetic properties as reported in the Table.

EXAMPLE 4 The magnet material used in Example 3 was oriented in a magnetic field of 4.000 ()e and die pressed instead of being isostatically compacted as was the material of Example 3. The die pressed compact was then sintered at a temperature of l925F for 30 minutes and cooled in a jet of helium from a temperature of approximately l450F. It may be noted that the Examples 3 and 4 differ with respect to the degree of orientation achieved which resulted in lower B,. and (BHL values for the magnet of Example 4.

EXAMPLE 5 The material used was of the composition: MM 16%. FM 16% and Co 68%. Powder of 2-l0 micron size was obtained by ball milling and to the powder a 15% by weight of sinter aid of Sm (60% Co (40%) was added. The compact was made by die-pressing at 85,000 psi in an orienting field of 4,000 Oe. The resulting compact was sintered at l920F for 45 minutes and fast cooled in helium gas jet from 1450F. The sintered magnet had the properties shown in the Table.

EXAMPLE 6 The magnet composition shown in Table l was obtained from a single melt of the reported CoSmMMFM composition. The material was oriented in a magnetic field of 4,000 Oe and compacted in a die press at 86.000 psi. The compact was sintered at 1920F for one hour and rapidly cooled in helium gas from the temperature of approximately l400F. The resulting magnet has a high H value of l2.000 Oe. This indicates that these magnets in accordance with the invention could be made alternately by blending MMJFM| J-C05 and a sinter aid containing samarium or by single melt incorporating the levels of samarium indicated in Table l.

As may be seen from Table l and the specific Examples. in addition to the conventional permanent magnet properties intrinsic coersive force H induction coercive force He. remanence B, and energy product (BHl new parameters are being used to describe magnetic and thermal stability and useful magnetic value of the rare-earth cobalt magnets to which this invention relates. One such parameter is H the limiting field for useful performance. H; in effect defines the onset of the knee of the intrinsic demagnetization curve. Also. in a recent stability classification, the ratio H /B, was established according to which i. magnets having high-stability magnetization,

l (HR/B 0.5 and ii. magnets having very high-stability magnetization with H /B, l.0 'D. J. lden. C. E. Ehrenfricd and H. 1. Garrett. Proc. (.onl'. Magnetism and Magnetic Materials. pp IOZh-lU-Jb. I972.

From Table I it is apparent that the magnets of the present invention meet both high and very high stability magnetization criteria. It was established that the higher the values of the stability parameters H and H /B the lower the irreversible losses which occur when magnets are exposed to elevated temperature during use in various applications. Another useful parameter describing the potential of these magnets in such applications as traveling wave tubes and other applications requiring high coercive force is the intrinsic energy product; thus the measure H X 8,. is coming into use. This value is 50 to 80 MGOe for the magnets of this invention compared with about to S0 for conventional magnets.

D. V. Ratnam and M. G, H. Wells. Cont". on Magnetism as Magnetic Materials. 1973. Properties of Mischmetal Cobalt Magnets" Further from the specific examples present in Table I. it may be seen that with the magnets of the invention constituting Examples I through 6 energy product values of 10 to l6 MGOe have been obtained with moderate orienting fields. compared to the orienting fields used with the prior-art magnets constituting Examples 7 through 9 of Table I. This is dramatically demonstrated by Examples 7 and 9. which exhibited lower energy product values than the magnets constituting Examples I through 5. despite the fact that these magnets had samarium contents in excess of 1071 and substantially higher orienting fields were employed. All of the magnets in Table I in accordance with the invention (Examples 1-6) have less than l07r samarium. Also. all the magnets in accordance with the invention have a stability parameter within the limits of 0.8 to 2 H /B I claim:

1. A permanent magnet having an energy product of 10 to 17 MGOe. a sintering temperature of 2000F max. and a stability parameter of 0.8 to 2 H,.-/B,.. said magnet being produced from compacted and sintered particles of a oRE compound where RE is a combination of rare-earth elements within the range of 32 to 42% by weight and balance cobalt. with said total combination of rare-earth elements consisting essentially of. in weight percent. up to 30 samarium. 25 to mischmetal, and 25 to 75 substantially cerium-free mischmetal containing less than about 3 cerium.

2. The magnet of claim I wherein the mischmetal and cerium-free mischmetal are present in approximately equal amounts.

3. The magnet of claim I wherein samarium is present within the range of 8 to 307: of total combination of rare-earth elements. 

1. A PERMANENT MAGNET HAVING AN ENERGY PRODUCT OF 10 TO 17 MGOE, A SINTERING TEMPERATURE OF 2000*F MAX. AND A STABILITY PARAMETER OF 0.8 TO 2 HK/BR, SAID MAGNET BEING PRODUCED FROM COMPACTED AND SINTERED PARTICLES OF A CORE COMPOUND WHERE RE IS A COMBINATION OF RARE-EARTH ELEMENTS WITHIN THE RANGE OF 32 TO 42% BY WEIGHT AND BALANCE COBALT, WITH SAID TOTAL COMBINATION OF RARE-EARTH ELEMENTS CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, UP TO 30 SAMARIUM, 25 TO 75 MISCHMETAL, AND 25 TO 75 SUBSTANTIALLY CERIUM-FREE MISCHMETAL CONTAINING LESS THAN ABOUT 3 CERIUM.
 2. The magnet of claim 1 wherein the mischmetal and cerium-free mischmetal are present in approximately equal amounts.
 3. The magnet of claim 1 wherein samarium is present within the range of 8 to 30% of total combination of rare-earth elements. 